Which loss function is the brain optimizing in order to learn advanced visual skills without expert/human supervision?
I think you are slightly confusing 2 problems. 1 being classification of meta visual elements and the other being the visual system itself.

Our visual system, when it comes to processing information, has had billions of years of iteration(training), so that at birth(and before), we are already tuned for the processing of visual stimuli, as well as have the mechanisms to decipher objects in our spatial field of view. 

These two papers(L1, L2), have a great deal of information about the evolution of our visual system and its processing. The second speculates on the connection of said evolution and the construction of "seeing systems" very interesting.

For further inquiry on this in particular, check out David Marr. He was probably the most influential early computer vision mind. He still is mentioned in many top-down AGI and computer vision research projects to this day.